Passive radiator (speaker)

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Passive radiator enclosure with front mounted passive radiator; back or side mounting is also used

A speaker enclosure using a passive radiator usually contains an "active driver" (or main driver), and a "passive cone" (or drone or passive radiator). The active driver is a regular driver, and the passive is typically the same or similar, but without a voice coil or magnet assembly. It is only a suspended cone, not attached to a voice coil or electrical circuit. The passive radiator usually has some means to adjust its mass, thereby allowing the speaker designer to change the box tuning. Internal air pressure produced by movements of the active driver cone moves the passive radiator cone as well.

Passive radiators are used instead of a reflex port for much the same reasons—to tune the small volume and driver for better low frequency performance. Especially in situations in which a port would be inconveniently sized (usually too long for practical box configurations). They are also used to eliminate port turbulence and reduce motion compression caused by high velocity airflow in small ports (especially small diameter ones). Passive radiators are tuned by mass variations (M_mp), changing the way their compliance interacts with motion of the air in the box. The weight of the cone of the passive radiator should be approximately equivalent to the mass of the air that would have filled the port which might have been used for that design. Passive radiators do not behave exactly as do (more or less) equivalent bass-reflex designs in that they cause a notch in system frequency response at the PR's free air resonant frequency; this causes a steeper roll-off below the system's tuned frequency F_b, and poorer transient response. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PRs to reflex ports. PR speakers, however, are more complex to design and likely to be more expensive as compared to standard reflex enclosures.

The frequency response of a passive radiator will be similar to that of a ported cabinet, with two exceptions. The system low frequency rolloff in a passive radiator design will be slightly steeper, and will have a notch (dip) in frequency response due to the V_as (compliance, or stiffness of the speaker cone) of the passive radiator. The goal in designing a passive radiator is to adjust the tuning so that this notch is below audible levels.

[edit] Misconceptions

In technical support circles, it has long been known that some owners of speaker cabinets featuring a passive radiator have taken the cabinet apart, and have come to the conclusion that someone (the manufacturer or salesperson) has taken advantage of them, by leaving out much of one of the drivers. They misconstrue the technology, and sometimes think the manufacturer put a fake speaker in the cabinet (since there is no magnet or wire) just to give the appearance of a larger more powerful speaker.